On 10/12/2020 07:53, Joonas Lahtinen wrote:
+ Tvrtko and Chris for comments
Code seems to be added in:
commit 0cd4684d6ea9a4ffec33fc19de4dd667bb90d0a5
Author: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Date: Tue Nov 21 18:18:50 2017 +0000
drm/i915/pmu: Add interrupt count metric
I think later in the thread there was a suggestion to replace this with
simple counter increment in IRQ handler.
It was indeed unsafe until recent b00bccb3f0bb ("drm/i915/pmu: Handle
PCI unbind") but now should be fine.
If kstat_irqs does not get exported it is easy enough for i915 to keep a
local counter. Reasoning was very infrequent per cpu summation is much
cheaper than very frequent atomic add. Up to thousands of interrupts per
second vs "once per second" PMU read kind of thing.
Regards,
Tvrtko
Quoting Thomas Gleixner (2020-12-06 18:38:44)
On Fri, Dec 04 2020 at 18:43, Jerry Snitselaar wrote:
Now that kstat_irqs is exported, get rid of count_interrupts in
i915_pmu.c
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -423,22 +423,6 @@ static enum hrtimer_restart i915_sample(struct hrtimer *hrtimer)
return HRTIMER_RESTART;
}
-static u64 count_interrupts(struct drm_i915_private *i915)
-{
- /* open-coded kstat_irqs() */
- struct irq_desc *desc = irq_to_desc(i915->drm.pdev->irq);
- u64 sum = 0;
- int cpu;
-
- if (!desc || !desc->kstat_irqs)
- return 0;
-
- for_each_possible_cpu(cpu)
- sum += *per_cpu_ptr(desc->kstat_irqs, cpu);
-
- return sum;
-}
May I ask why this has been merged in the first place?
Nothing in a driver has ever to fiddle with the internals of an irq
descriptor. We have functions for properly accessing them. Just because
C allows to fiddle with everything is not a justification. If the
required function is not exported then adding the export with a proper
explanation is not asked too much.
Also this lacks protection or at least a comment why this can be called
safely and is not subject to a concurrent removal of the irq descriptor.
The same problem exists when calling kstat_irqs(). It's even documented
at the top of the function.
Thanks,
tglx
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